1. Field of the Invention
The present invention relates to a light source apparatus that can change the oscillation wavelength and an image pickup apparatus using the light source apparatus.
2. Description of the Related Art
Various light sources, especially, laser light sources that can change the oscillation wavelength have been used in the fields of communication networks and inspection apparatuses.
There are demands for high-speed wavelength switching in the field of communication networks and high-speed and wide-range wavelength sweeping in the field of inspection apparatuses.
Examples of inspection apparatuses to which wavelength-tunable (sweep) light sources are applicable are a laser spectroscope, a dispersion measuring apparatus, a film-thickness measuring apparatus, and a swept source optical coherence tomography (SS-OCT) apparatus, among others.
Optical coherence tomography (OCT) is an imaging technique in which a tomographic image of a specimen is obtained by using optical interferometry. Thus, at the heart of an OCT system is an interferometer illuminated by a broadband light source. Recently, there has been an increased interest in OCT, and abundant studies thereof have been performed in the medical field because of OCT's high spatial resolution (in the order of microns) and noninvasive imaging methods.
In SS-OCT, spectral interferometry is used to acquire depth information, but a spectroscope is not used. Hence, this technique reduces the loss of light, and is expected to acquire an image at a high signal-to-noise (SN) ratio.
In a medical image pickup apparatus to which the SS-OCT technique is applied, the image acquisition time decreases as the sweep speed increases, and the spatial resolution of a tomographic image increases as the wavelength sweep width increases. Hence, the parameters of the light source are central to obtaining an appropriate image.
More specifically, in OCT, the depth resolution is given by the following Expression (1):
                                          2            ⁢                                                  ⁢            ln            ⁢                                                  ⁢            2                    π                ×                              λ            0            2                                Δ            ⁢                                                  ⁢            λ                                              (        1        )            where Δλ represents the wavelength sweep width and λ0 represents the oscillation wavelength of the light source. Therefore, to increase the depth resolution, it is necessary to increase the wavelength sweep width, and a wide-band wavelength swept light source is demanded.
As a light source to be used in the SS-OCT apparatus, a light source based on a dispersion tuning method that changes the wavelength by using wavelength dispersion (also simply referred to as dispersion) of the refractive index in a resonator has been studied for the band mainly used in the communication field, as is disclosed in S. Yamashita, et al., Optics Express, Vol. 14 (2006), p. 9299 (hereinafter referred to as Non-Patent Document 1).
In the dispersion tuning method, the oscillation wavelength in an active mode-locked state is controlled by utilizing the fact that the free spectral range (FSR) of the resonator has a wavelength dependency. That is, since wavelength sweeping is performed by changing the frequency of a modulation signal that brings about active mode locking, high-speed wavelength sweeping can be performed by changing the frequency of the modulation signal at high speed.
Here, the free spectral range indicates the frequency interval of resonator modes for light circulating in the resonator. The free spectral range (FSR) is given by the following Expression (2):
                    FSR        =                  c          nL                                    (        2        )            where c represents the speed of light in a vacuum, n represents the refractive index of the resonator, and L represents the resonator length.
The dispersion tuning method utilizes the wavelength dependency of the FSR, and sweeps the center wavelength in a mode-locked state by sweeping the mode locking frequency.
Non-Patent Document 1 describes that the wavelength sweep range Δλ in dispersion tuning is given by the following Expression (3):
                              Δ          ⁢                                          ⁢          λ                =                  n          cDN                                    (        3        )            where n represents the refractive index of the resonator, D represents the dispersion parameter of the resonator, and N represents the order of mode locking (natural number).
In contrast, Japanese Patent Laid-Open No. 7-307512 (hereinafter referred to as Patent Document 1) discloses a laser apparatus in which a saturable absorber is located in the center of a Fabry-Perot optical resonator and a mode-locked operation is obtained by applying optical clock pulses to the saturable absorber in order to stably generate short optical pulses.
In wavelength sweeping using the dispersion tuning method disclosed in Non-Patent Document 1, the wavelength sweep velocity can be increased by changing the frequency of the modulation signal at high speed. However, a plurality of modes having a fixed phase relationship are simultaneously excited in the mode-locked laser, the spectral width (linewidth) of the oscillation spectrum is easy to increase. Hence, this method cannot always sufficiently respond to applications that need a narrow spectral width.
In the laser apparatus disclosed in Patent Document 1, optical pulses propagate in the right and left directions symmetrically with respect to the saturable absorber in the optical resonator and collide with each other at the saturable absorber, and optical clock pulses are simultaneously input to the saturable absorber. Patent Document 1 describes that this laser apparatus can reduce the loss of the saturable absorber to obtain a stable operation of the laser and that an ultrahigh-speed train of short optical pulses are obtained because of mode locking using the optical clock pulses. However, this laser apparatus can obtain short optical pulses, but is not designed to narrow the oscillation spectrum linewidth.